As for theories, it’s not quite out of the realm of possibility. Aemond seems to treat Helaena better than his brother, and given Aegon’s self-destructive behavior, it makes sense that Helaena would seek comfort and companionship with someone else.
But as others pointed out in the comments of the original tweet, this clip is not enough to support this theory. Sure, some argue that this is why Aemond just waltzes into Helaena’s room and why he later claims to be next in line for the throne after Aegon instead of Aegon’s son, but as of now, a secret relationship between Aemond and Helaena is more based on conjecture than what the show has presented to us so far.
Personally, I can’t hear the twins say “Aemond” clearly enough in the clip to give this theory my full support, but in an effort to ignore the fact that he’s also her brother, I can see why fans instead use Aemond and Helaena. would send from Aegon and Helaena. Aegon sexually assaulted a girl in the previous episode “The Lord of the Tides” and spent much of their family dinner making sexually suggestive comments towards their cousin Baela (Bethany Antonia). As rotten as Aemond is as a person (e.g. the Vhagar incident), Helaena doesn’t have many other options unfortunately.
Helaena earns better than Aemond and Aegon, but unfortunately for her the Targaryens; propensity for incest and Alicent and Otto’s desire to hold onto power for the Hightowers means that those two are really her only options if she wants to avoid the criticism Rhaenrya has endured regarding her children’s paternity. It’s not that she doesn’t deserve the same sexual liberties that Rhaenyra has enjoyed, but rather that she’s even more trapped in her situation than Rhaenyra was.
If Aemond really is the father of Helaena’s children, then that could be a potential weakness for Rhaenyra and her followers to exploit as they fight for her rightful place on the Iron Throne. It’s even harder for Alicent to be morally high and argue for Aegon’s right to the throne when the paternity of her own grandchildren can be questioned.